The oldest solids known to have formed within the Solar System are the CAI's. Pb-Pb and Al-Mg dating techniques yield absolute and relative ages, respectively, that are consistent with an age spread of \(\le \)  0.3 Myr, and that is generally used to define t = 0 for the Solar System [1]}, [2]}, [3]}, [4]}. CAI's are likely to have condensed from the solar nebula at a time when it was still hot from accretion, a time which is only expected to have lasted for \(\sim \) 0.1 Myr [5]}. CAI's are highly refractory and fortunately many have survived, embedded in chondrites. [6]} have shown that they formed in a reducing atmosphere at a pressure and temperature consistent with expectation for a protoplanetary disk dominated by H.
